1. Field of the Invention
The present invention relates to a method of preparing a cracking catalyst which comprises active catalytic fines dispersed in a matrix and the use thereof in a catalytic cracking process whereby the amount of sulfur compounds in the catalyst regenerator flue gas is reduced.
In fluidized catalytic cracking (FCC) systems a fluidized bed of particulate catalyst is continuously cycled between a cracking zone wherein the hydrocarbon feedstream is contacted with the fluidized catalyst particles and a catalyst regeneration zone. Hydrocarbon cracking in the reaction zone results in deposition of carbonaceous coke on the catalyst particles. The cracked hydrocarbons are thereafter separated from the coked catalyst and withdrawn. The coked catalyst is then stripped of volatiles and passed into the catalyst regeneration zone. In the catalyst regenerator, the coked catalyst is contacted with a gas containing a controlled amount of molecular oxygen to burn off a desired portion of coke and simultaneously heat the catalyst to a sufficiently high temperature so as to activate the catalyst when the catalyst is again contacted with the hydrocarbon stream in the cracking zone for catalyzing the hydrocarbon cracking. A flue gas is formed by the burning procedure in the regeneration zone and is normally passed into the atmosphere after treatment to remove particulates and carbon monoxide.
The hydrocarbon feeds processed in commercial FCC units normally contain sulfur. A significant portion of the sulfur contained in the hydrocarbon feedstream processed in an FCC system is invariably deposited on the catalyst particles in the coke. Accordingly, the flue gas which is formed by burning the coke in the catalyst regenerator contains gaseous sulfur oxide (SO.sub.x) such as sulfur dioxide and sulfur trioxide.
At present, there are restrictions on the quantity of SO.sub.x which can be emitted from FCC units and these restrictions are being reviewed for the possibility of making such restrictions more stringent. Currently available methods of reducing SO.sub.x emissions from FCC units and, in particular the catalyst regenerator, include desulfurizing the regenerator flue gas by conventional stack gas scrubbing which necessitates large capital investments. Alternatively, desulfurizing the hydrocarbon feed in a separate desulfurization unit to reduce SO.sub.x emissions also requires elaborate additional processing operations and necessitates substantial additional capital. On the other hand, most of the sulfur which is contained in the hydrocarbon feedstock does not become sulfur which is contained within the coke deposited on the catalyst. Instead, the sulfur is converted to normally gaseous sulfur compounds such as hydrogen sulfide. The sulfur compounds are conventionally removed from the reactor along with the fluid hydrocarbon products. Removal of sulfur compounds such as hydrogen sulfide from the product effluent from a fluidized reactor is relatively simple and inexpensive as compared to the methods required for removal of sulfur oxides from the regenerator flue gas and hydrocarbon feed. However, many of the petroleum stocks currently available for processing in fluidized units have a high sulfur content and thus, the large capital investments required to reduce SO.sub.x emissions from the regenerator flue gas must be made in order to process hydrocarbon streams.
Catalyst modification, therefore, to effect the required reductions in SO.sub.x emissions from fluidized cracking units is much preferred over the currently available methods.
Cracking catalysts are solid materials which have acidic properties. Because of the nature of the reactions taking place, the catalyst must have high porosity. Furthermore, since the catalyst circulates rapidly between reaction zones and burning, or regeneration zones, it must also have resistance to abrasion, temperature changes and the like.
Natural catalysts which are used in fluidized systems are composed primarily of silica and alumina, but they contain certain other materials which may be harmful under certain circumstances. The synthetic crystalline silica-alumina materials (active crystalline zeolites) are generally made from pure materials so that many of the shortcomings of the natural materials have been overcome. However, zeolite catalysts contained in a gelled matrix such as a silica-alumina matrix and prepared by prior art processes are subject to excessive attrition, aging and loss of activity and selectivity. Accordingly, the trend in commercial fluid catalytic cracking is toward the use of high density catalysts. The higher density in conjunction with increased attrition resistance is a major factor in improving catalyst retention in a fluidized cracking unit. As a result, the make-up rate of fresh catalyst can be reduced and dust emissions from the flue gas stacks are lowered. At the same time, however, the characteristic of high density must not result in the loss of product selectivity.
2. Brief Description of the Prior Art
It has been suggested in U.S. Pat. No. 3,835,031 to reduce the amount of sulfur oxides in FCC regenerator flue gas by impregnating a Group II-A metal oxide onto a conventional silica-alumina cracking catalyst. The Group II-A metals react with sulfur oxides in the flue gas to form solid sulfur-containing compounds. It has been disclosed that the attrition encountered when using unsupported Group II-A metals is thereby reduced. It has also been disclosed, however, that Group II-A metal oxides, such as magnesia, when used as a component of cracking catalysts, have a highly undesirable effect on the activity and selectivity of the cracking catalyst. The addition of a Group II-A metal to a cracking catalyst results in two particularly noticeable adverse consequences relative to the results obtained without the Group II-A metals: (1) the yield of the liquid hydrocarbon fraction is substantially reduced, typically by greater than one volume percent of the feed volume; and (2) the octane rating of the gasoline is substantially reduced. Both of the above adverse consequences are severely detrimental to the economic viability of a fluidized operation, and even complete removal of sulfur oxides from regenerator flue gas could not compensate for the losses in yield and octane which result from adding Group II-A metals to an FCC catalyst.
Other patents including U.S. Pat. Nos. 4,071,436; 4,115,249; 4,115,250; 4,115,251; 4,166,787; 4,204,944; and 4,204,945 propose the reduction of SO.sub.x emissions from regenerator flue gas by contacting the sulfur compounds with an oxidation promoter and reactive alumina. The resulting solid aluminum-sulfate compound, formed in the regenerator, is reduced to hydrogen sulfide gas and reactive alumina in the reaction zone. The alumina can be impregnated on the standard catalyst, incorporated into the catalyst during manufacture, or admixed with a standard catalyst as a separate particle. There is a significant economic advantage by shifting the sulfur from the regenerator flue gas to the reactor effluent as hydrogen sulfide. As mentioned previously, most of the sulfur contained in the hydrocarbon feedstock is converted to normally gaseous sulfur compounds such as hydrogen sulfide in the reactor. Accordingly, if gaseous sulfur compounds normally removed from the fluidized unit in the regenerator flue gas can instead be removed from the reactor as hydrogen sulfide along with the processed hydrocarbons, the shifted sulfur is simply a small addition to the large amount of hydrogen sulfide and organic sulfur already present in the reactor effluent. Hydrogen sulfide removal from the reactor effluent is substantially less expensive than separate feed desulfurization or flue gas desulfurization.
Co-pending application Ser. No. 945,967, filed Sept. 26, 1978 now abandoned, discloses impregnating standard catalysts with rare earth oxides such as Cr.sub.2 O.sub.3, MnO, or CoO, alone or in combinations, or rare earth oxide with a platinum group metal oxidation promoter to reduce the sulfur content of coke and shift the sulfur removal to the cracked hydrocarbon effluent.
While the above-mentioned disclosures teach methods of reducing SO.sub.x emissions by catalyst modification, the disclosed processes require the addition of elements or compounds to the cracking catalyst. The present invention does not require the addition of any element or compound to the catalyst to reduce sulfur emissions and shift sulfur recovery from the regenerator flue gas to the effluent leaving the reaction zone. The reduction of sulfur emissions is accomplished solely by the manner in which the catalyst is formed.
Virtually all fluidized catalysts presently used include a gel or clay-type matrix of silica-alumina in which is dispersed particles of the crystalline zeolitic catalytic material. One method used commercially for manufacturing the catalysts involves formation of a silica-alumina co-gel, addition of small particles of zeolite to the co-gel, and formation of catalyst particles by spray-drying.
Several U.S. Patents disclose altering various characteristics of zeolite-containing silica-alumina catalysts by forming the cracking catalysts in a particular manner. For example, U.S. Pat. No. 4,219,446 discloses producing an attrition resistant, more active, more selective and more stable zeolite-containing catalyst by specially preparing a zeolite-containing silica-alumina hydrogel. One of the critical aspects of the invention is nozzle-mixing an acid alum solution and an aqueous solution of sodium silicate which comprises finely dispersed kaolin clay and calcined rare earth exchanged zeolite Y. The mixture has a pH above 9.
U.S. Pat. No. 3,957,689 discloses a process for preparing an attrition resistant zeolite hydrocarbon conversion catalyst comprising the steps of decreasing the pH of a sodium silicate solution to a pH of 2.0-3.2 by adding a mixed sulfuric acid-aluminum sulfate solution to form a buffered silica sol, adding clay before, during or after sol formation, preparing a water slurry of a crystalline zeolite and adjusting the pH to about 3-5, mixing the slurry with the buffered silica sol-clay slurry to prepare a spray drier feed slurry.
U.S. Pat. Nos. 3,520,828 and 3,939,058 disclose methods of preparing the silica and alumina matrix components of a fluid cracking catalyst which comprises the step of nozzle mixing a sodium silicate solution and an acid alum solution to form a hydrosol containing crystalline aluminosilicate fines. The gelled hydrosol has a pH of about 9.0.